The invention pertains to a process for the production of partially reduced sputtering targets for cathodic evaporation based on powder mixtures of indium oxide and tin oxide by means of pressure-assisted sintering.
Oxide ceramic targets of indium oxide-tin oxide (ITO) are used for the production of thin, transparent, electrically conductive layers by cathodic evaporation (sputtering). Layers of this type find application primarily in flat-screen technology. The thin indium oxide-tin oxide layers can be produced either by the sputtering of metal targets in a reactive oxygen atmosphere or by the sputtering of oxide ceramic targets.
The sputtering process with oxide ceramic targets has the advantage that the oxygen flow in the sputtering chamber can be kept low, which makes it easier to control the sputtering process. When metal targets are used, the oxygen flow must be high, which makes process control more difficult. In comparison with completely oxidized targets of In.sub.2 O.sub.3 +SnO.sub.2, targets which have a substoichiometric oxygen content offer the advantage that the oxygen content crucial to the conductivity of layer can be adjusted over a wider range during the deposition of the layer by sputtering. In addition, the working range, that is, the range of optimum layer conductivity as a function of the oxygen flow, is wider with these partially reduced targets than it is in the case of fully oxidized targets.
According to U.S. Pat. No. 4,690,745, indium oxide-tin oxide powder mixtures are hot-pressed under reducing conditions at 850.degree.-1,000.degree. C., during which the oxides are hot-pressed in a graphite hot-press mold or with the addition of carbon or carbon-releasing organic material. During the pressing step, the oxides are partially reduced, so that a target is created which contains less than the stoichiometric amount of oxygen.
The targets which can be obtained by means of this process have densities of &lt;90% of the theoretical density (TD); they are also characterized by poor electrical conductivity (.rho.=0.1-0.6 .OMEGA.-cm) and insufficient mechanical stability. All three of these factors have a negative influence on the use of such targets for sputtering. In addition, it is difficult to adjust the degree of reduction in this process in a manner which is both reproducible and homogeneous over the entire extent of the target.
DE 41 24 471 describes a process for the production of ITO targets in which, through the use of pre-reduced powder, targets can be produced with a very uniform degree of reduction by means of hot-pressing or hot isostatic pressing. It has been found, however, that the step of reducing the powder is extremely time-consuming and cost-intensive. In addition, a great deal of effort is required to achieve a sufficient degree of reproducibility in the degree of reduction.